1. Field of the Invention
The invention relates to a hydraulic transaxle, which incorporates a hydraulic motor for driving a single wheel, and is steerably supported by a vehicle chassis. The invention also relates to a four-wheel driving vehicle including the hydraulic transaxles for driving respective wheels.
2. Related Art
As disclosed in U.S. Patent Application Publication No. 2003/0106725A1, there is a conventional vehicle having left and right steerable wheels linked to a steering operation device such as a steering wheel. In the conventional vehicle, according to steering, e.g., according to increase of a rotational angle of a steering wheel from its straight traveling position, turning angles of the left and right steerable wheels are increased so that the turning angle of the steerable wheel on the turning inside of the vehicle (hereinafter referred to as “inside wheel”) becomes larger than that of the steerable wheel on the turning outside of the vehicle (hereinafter referred to as “outside wheel”).
This vehicle can turn on a small circle by a reduced steering degree (a small rotational angle of the steering wheel). However, the steerable wheels are not drivingly connected to a prime mover. The rotary speeds of the steerable wheels depend upon the speed control of (unsteerable) drive wheels by an operator's manipulation. Such a vehicle is disadvantageous in traveling capacity on a bad road and climbing capacity.
As disclosed in Japanese Utility Model Application Publication No. Sho 58-58932 and Japanese Utility Model Publication No. Sho 62-37775, there is a conventional four-wheel driving vehicle having four fixed displacement hydraulic motors for driving respective four wheels, i.e., left and right front wheels and left and right rear wheels. The left and right front wheels are steerable wheels, which are steered by turning of a steering wheel. The hydraulic motors are mounted on a vehicle chassis and drivingly connected to the respective wheels through respective deceleration gear trains in respective housings.
The vehicle has left and right variable displacement hydraulic pumps. The left front and rear hydraulic motors are fluidly connected in parallel to the left hydraulic pump, and the right front and rear hydraulic motors to the right hydraulic pump. The front hydraulic motors can be fluidly separated from the respective hydraulic pumps so as to put the vehicle into a two-wheel drive mode. By operating a speed control manipulator, the left and right hydraulic pumps are synchronously controlled in their delivery direction and amount so as to control the traveling direction and speed of the vehicle. The hydraulic pumps are fluidly connected to each other so as to allow differential rotation of left and right rear wheels (and left and right front wheels). The vehicle is also provided with a valve for differential locking, i.e., cutting off the fluidal connection between the hydraulic pumps, so as to facilitate escape of the vehicle from mud or a ditch.
This conventional vehicle is disadvantageous in minimization because the hydraulic motors and the respective deceleration gear train housings are mounted upright on the vehicle chassis. The vehicle is also disadvantageous in cost because of the two hydraulic pump. Further, when the vehicle travels in the four-wheel drive mode, the front and rear hydraulic motors on each of left and right sides of the vehicle are fluidly connected in parallel to the common left or right hydraulic pump, so that the amount of hydraulic fluid supplied to each of the hydraulic motors is restricted. Therefore, the vehicle is disadvantageous in efficiency of acceleration, i.e., high-speed traveling.
As disclosed in Japanese Patent Application Publication No. 2004-210215, there is another conventional vehicle having a transaxle housing incorporating left and right hydraulic motors for driving respective left and right wheels. The left and right wheels are supported by respective left and right wheel support units steerably supported on left and right ends of the transaxle housing, and drivingly connected to the respective hydraulic motors. The left and right hydraulic motors are fluidly connected in parallel to a common hydraulic pump so as to differentially drive the left and right wheels. The left and right hydraulic motors may be variable in displacement. In this case, means for controlling the displacements of the hydraulic motors, e.g., movable swash plates, are operated according to steering.
The transaxle housing incorporating the left and right hydraulic motors is laterally extended in the vehicle between the left and right wheel support units so as to restrict a space for other parts or assemblies in the vehicle. Further, the large transaxle including the transaxle housing and the left and right wheel support units is not handy, and is disadvantageous in ensuring a free space in a small size vehicle.